Organic Chemistry, 5th Edition L.G. Wade, Jr Chapter 16 Aromatic Compounds Jo blackburn Richland college, dallas TX Dallas County Community College District c 2003. Prentice hall
Chapter 16 Aromatic Compounds Jo Blackburn Richland College, Dallas, TX Dallas County Community College District © 2003, Prentice Hall Organic Chemistry, 5th Edition L. G. Wade, Jr
Discovery of benzene Isolated in 1825 by Michael Faraday Who determined c h ratio to be 1: 1 Synthesized in 1 8 34 by eilhard Mitscherlich who determined molecular formula to be C6H6 Other related compounds with low C: H ratios had a pleasant smell, so they Were classified as aromatic Chapter 16
Chapter 16 2 Discovery of Benzene • Isolated in 1825 by Michael Faraday who determined C:H ratio to be 1:1. • Synthesized in 1834 by Eilhard Mitscherlich who determined molecular formula to be C6H6 . • Other related compounds with low C:H ratios had a pleasant smell, so they were classified as aromatic. =>
Kekule structure after multiple bonds were suggested,SA Proposed in 1866 by Friedrich Kekule, shortly Failed to explain existence of only one isomer of 1.2-dichlorobenzene H H H H H H Chapter 16
Chapter 16 3 Kekulé Structure • Proposed in 1866 by Friedrich Kekulé, shortly after multiple bonds were suggested. • Failed to explain existence of only one isomer of 1,2-dichlorobenzene. C C C C C C H H H H H H =>
H Resonance structure Each sp hybridized c in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring 1.397A H H 120° H 120° Chapter 16
Chapter 16 4 Resonance Structure Each sp2 hybridized C in the ring has an unhybridized p orbital perpendicular to the ring which overlaps around the ring. =>
Unusual reactions Alkene+ KMnO4>diol (addition) Benzene Kmno,-> no reaction Alkene+ Br2/CCl4>dibromide(addition) Benzene br/ccl-> no reaction With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene hbr (substitution! ) Double bonds remain > Chapter 16
Chapter 16 5 Unusual Reactions • Alkene + KMnO4 → diol (addition) Benzene + KMnO4 → no reaction. • Alkene + Br2 /CCl4 → dibromide (addition) Benzene + Br2 /CCl4 → no reaction. • With FeCl3 catalyst, Br2 reacts with benzene to form bromobenzene + HBr (substitution!). Double bonds remain. =>
Unusual Stability Hydrogenation of just one double bond in benzene is endothermic (85.8 predicted) 36 kcal (57.2 predicted) resonance energy energy +1. 8 kcal resonance energy 574 kcal 55.4 kcal 49.8 kcal 28.6 energy
Chapter 16 6 Unusual Stability Hydrogenation of just one double bond in benzene is endothermic! =>
H Annulenes All cyclic conjugated hydrocarbons were proposed to be aromatic However, cyclobutadiene is so reactive that it dimerizes before it can be isolated And cyclooctatetraene adds br read lily Look at Mo s to explain aromaticity Chapter 16
Chapter 16 7 Annulenes • All cyclic conjugated hydrocarbons were proposed to be aromatic. • However, cyclobutadiene is so reactive that it dimerizes before it can be isolated. • And cyclooctatetraene adds Br2 readily. • Look at MO’s to explain aromaticity. =>
MO Rules for benzene Six overlapping p orbitals must form six molecular orbitals Three will be bonding three antibonding LoWest energy MO will have all bonding interactions, no nodes As energy of Mo increases, the number of nodes increases Chapter 16
Chapter 16 8 MO Rules for Benzene • Six overlapping p orbitals must form six molecular orbitals. • Three will be bonding, three antibonding. • Lowest energy MO will have all bonding interactions, no nodes. • As energy of MO increases, the number of nodes increases. =>
H MO’ s for benzene all antIbonding node 4 node antibonding no node bonding -node丌 all bonding
Chapter 16 9 MO’s for Benzene =>
Energy Diagram for Benzene The six electrons fill three bonding pi orbitals All bonding orbitals are filled (closed shell) an extremely stable arrangement energy T nonbonding li 10
Chapter 16 10 Energy Diagram for Benzene • The six electrons fill three bonding pi orbitals. • All bonding orbitals are filled (“closed shell”), an extremely stable arrangement. =>